The project is a joint venture between the World Wildlife Fund (WWF) and a South African group called Ezemvelo KZN Wildlife.

"Tight security for black rhino is essential but it's only one part of the solution," WWF spokesman Dr Jacques Flamand said.

Wiped out

"The other part is ensuring that black rhino numbers increase as fast as possible in order to reduce the threat of extinction from possibilities such as increased poaching, drought, flood and disease."

Extreme poaching virtually wiped out the rhinos in the 1970s and 1980s. At the worst point there were just 2,500 of them left in the world, which meant 96% of the entire population had been killed.

Calves

This group of rhino is the second sent out to new reserves by the project leaders. The first group, which was released in 2004, has been doing very well.

"There have been no losses through fights or accidents," a Dr Flamand said.

He added they were hoping that some baby rhino, called calves, would be born into the first group soon too.